There are severe problems in economically manufacturing ozone at levels up to about ten pounds per day. Conventional corona discharge ozone generation equipment suffers from the disadvantage that extensive feed air pretreatment is required or pure oxygen must be utilized. Also, ozone concentrations much over four percent are not economically obtainable by prior corona discharge technology.
Methods to produce ozone include thermolysis in a plasma above 2760.degree. C. followed by quenching in liquid oxygen; slow oxidation of phosphorus, beta irradiation of oxygen in nuclear reactors; high current electrolysis of aqueous phosphate solutions at room temperature: electron-beam irradiation of air; oxidation of yellow phosphorus in air to phosphorus sesquioxide and a small amount of ozone: UV irradiation of oxygen or air; and corona discharge. Of the above methods, only UV irradiation and corona discharge are of any commercial importance.
Ultraviolet ozone generation technology is frequently used for capacity requirements of under one pound per day; however, such method suffers from the disadvantages of high power consumption and low ozone concentration.
For example, the irradiation of air at 184.9 nm by one UV (4OW) bulb can produce about 0.5 g/h of ozone and a 0.25wt % maximum concentration. These maximum ozone yields and concentrations cannot be obtained simultaneously by the method. Ultraviolet ozone generators are used for food preservation, in brewery cellars, and in hotel and hospital air ducts. Thus, the low concentrations of ozone that are available from UV generators preclude them from being used for the treatment of water, because the transfer efficiencies of ozone from the air into water are low and large volumes of feed gas must be handled. More than 44 kW.h are required to generate 1 kg of ozone from dry air by light under high gas-flow rates and low concentrations.
Larger quantities and higher concentrations of ozone dictate the use of corona discharge technology. A corona discharge is a silent electrical discharge which is used to accelerate electrons so as to give them sufficient kinetic energy to split the oxygen-oxygen double bond upon impact with the oxygen molecule. The two oxygen atoms, which are formed from this collision, react with another oxygen molecule to form ozone. The thermodynamics of the ozone synthesis reaction is:
30.sub.2 .fwdarw.20.sub.3 .DELTA.H.degree..sub.f =144.8 kJ/mol (34.61 kcal/mol)
Therefore, the formation of 1 kg of ozone requires 3.02 MJ (721 kcal). A typical corona discharge ozone generator requires about 16.5 kW.h of electric energy to produce 1 kg of ozone from air at a 1wt % concentration. Thus, only about 5% of the electric energy which is supplied to the ozone generator is consumed in the generation of ozone. The largest portion of the electric energy appears as heat, and an insignificant amount is liberated as light energy.
Ozone decomposes thermally to oxygen; therefore, an ozone generator must be cooled very efficiently to prevent the generator from operating at elevated temperatures. In the event that heat is not adequately dissipated from the ozone generator, a portion of the ozone which is generated is simultaneously destroyed.
In corona discharge a substantial fraction of the electrical energy is converted to heat. The low volume of gas flowing between the electrodes does not have sufficient capacity to remove this heat. Thus, some external heat sink is necessary, since the decomposition of ozone is accelerated by increasing temperature. An example of a modern plate-type, air-cooled ozone generator is the Lowther cell. In other commercial ozone generators, the ozonated gas is water-cooled. The cooling process of these earlier ozone generators, is inefficient due to their configurations, thereby limiting their ability to convert air or pure oxygen to desirable concentrations.
A process and device for the generation of ozone by electrolytic processes is disclosed in U.S. Pat. No. 4,54l,989 (Sept. 17, 1985) to Foller. In this process, oxygen from air is cathodically reduced to an air electrode to form water, which in turn is decomposed to an inert anode to form ozone. U.S. Pat. No. 4,016,060 (Apr. 5, l977) to Lowther, discloses a method for increasing the electrical efficiency of a corona discharge reaction system. In Lowther, the corona is produced in a gas-filled gap between opposing electrodes by a high voltage, narrow pulse electrical discharge. In U.S. Pat. No. 4,176,061 (Nov. 27, l979) to Stopka, an apparatus and method for purifying liquids such as water is disclosed. More particularly, U.S. Pat. No. 4,176,061 discloses a gas having a high concentration of ozone provided by a generator comprising a plurality of electrodes connected in a series by gas flow lines. The voltage necessary to cause the corona discharge is applied externally and subject to contact by an operator of the apparatus.
In sum, the currently available commercial ozone generators, particularly those using corona discharge are expensive and lack efficiency in obtaining high concentrations of ozone.